1. Technical Field
The present invention concerns a method and a device in connection with the use of a wind turbine offshore, comprising a wind turbine connected via a shaft to a generator, which is rotationally mounted on a tower, and a foundation underneath in the form of a float or buoyancy element on which the tower is mounted.
2. Description of the Related Art
Wind turbines are increasingly being installed offshore, partly on account of space requirements and also to achieve optimally constant, exploitable wind conditions (higher mean speed, lower turbulence, thinner interface than on land). At present, they are mainly installed in shallow water where they can easily be placed on a foundation that stands on the sea-bed. Such installations require that sufficient shallow water areas are available. Along most of the coasts of the world and particularly along the coast of Norway, the water is generally too deep to allow wind turbines to be installed on the sea bed. The installation of wind turbines in shallow water can also cause problems for the vessels that are to carry out the installation. Most installation vessels will have a draught that is too great to allow them to operate in depths up to 10 m.
For these reasons, the use of floating supporting structures is a relevant solution. To make this financially interesting, each turbine must have a high capacity, for example in the order of 5 MW. With such a large output and by exploiting the wind properties offshore, it is expected that floating supporting structures will be able to compete on energy price with land-based installations.
The prior art concepts for floating supporting structures are usually based on a single floating structure (for example a vertical column) that is anchored to the sea bed by means of vertical stays (tethers). Other hull concepts are based on the technology for semi-submersible platforms. These have been specially developed to have favorable (small) motions in waves. A common feature of most of the prior art wind turbine concepts is that the aim is to restrict the motion of the platform as much as possible. Moreover, they are designed so that they can withstand extreme sea conditions. The stricter the requirements made for the motion, the greater the forces experienced in an extreme situation. The combinations of these requirements are therefore expensive and contribute to prior art sea-based wind turbine solutions generally being unprofitable to date.